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Breathe Slowly

London pea-soupers and the “great smog” of 1952 led to the Clean Air Act of 1956. In the 1980s, Mexico City was talked about as the place with the most polluted air. In recent years, Beijing and Delhi have often had terrible air quality. How do the levels of pollution in these different cities at these different times compare? Were any clearly more dangerous than the others?

Paul Singer, London, UK

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